The 33-year-old television personality cheered on the 27-year-old Cleveland Cavaliers player as the team took on the Toronto Raptors at Quicken Loans Arena, according to TMZ. The Cavaliers won 105 to 103.
Kardashian was spotted wearing oversized sunglasses and a leopard-print ensemble. E! News reported the star attended the game with friends.
"Khloe was at the game with a group of people. She was cheering, [doing] a lot of clapping, but she never stood to cheer," a source said.
The outing follows reports Thompson cheated during Kardashian's pregnancy. The television personality gave birth to daughter True Thompson on April 12, and has yet to address her relationship status with Thompson.
"She's just concentrating on that -- being a mom, her baby," her mom, Kris Jenner, said on The Ellen DeGeneres Show last week. "She's so excited about motherhood and trying to get the nursing thing down, which is a little tricky."
"She's figuring it out one day at a time," Jenner added. "She's just the best mom already and that baby is so cute, little True."
Kardashian said in a post on her website last week that she's looking forward to her first Mother's Day as a mom.
"This Mother's Day will be the most special yet!" she said. "I still can't believe I'm a mom -- it feels like such an honor. Now it feels even more important to honor all the amazing women in my family!"